1. Field of the Invention
This invention relates generally to the use of a colorant having a poly(alkyleneoxy) substituent to color thermoplastic resins containing an opacifier. In particular, the colorant may be incorporated with an opacifier into a semi-crystalline resin which can then be manufactured to close dimensional tolerances without experiencing shrinkage or warpage attributable to the colorant
2. Prior Art
Traditionally, pigments have been incorporated into thermoplastic resins, such as alpha-olefins, to impart color to the resin. Pigments offer the features of low cost, availability in a wide range of hues and exceptional lightfastness. However, at typical color loadings, pigments tend to opacify thermoplastic resins, which may be undesirable for some applications. Recent developments in clarifiers for thermoplastic resins, such as the dibenzylidene sorbitols disclosed in Hamada, et al., U.S. Pat. No. 4,016,118, have expanded the market for resin colorants which do not detract from the transparency of the final product. It has been found that thermoplastic resins colored with poly(alkyleneoxy) substituted chromophores show improved clarity over those colored with pigments. The use of poly(alkyleneoxy) substituted chromophores to color resins is disclosed in Baumgartner et al , U.S. Pat. No. 4,640,690 and Baumgartner et al., U.S. Pat. No. 4,732,570. In addition to providing coloration without sacrificing clarity, the aforementioned colorants demonstrate low extractability and migration as well as processing advantages due to their availability in liquid form at ambient temperatures.
Despite the advantages of employing poly(alkyleneoxy) substituted colorants in thermoplastic resin systems, their expense has limited their use to applications in which a premium is placed on the clarity of the final product. For example, in applications such as overcaps, threaded and snap-on lids, referred to generally as closures, opacifiers are incorporated into the resins, primarily for aesthetic reasons. Since the clarity of the final formed product is not pertinent when opacifiers are employed, pigments have been universally used to color closures.
Closures are often required to meet strict dimensional tolerances, especially when the closures contain moving parts such as may be used on shampoo or dish detergent bottles. One of the difficulties of manufacturing parts from thermoplastic resins has been the tendency of the resin to shrink or warp following formation of the part. Shrinkage is believed to be caused in part by the presence of pigments which act as nucleating agents in resins which are semi-crystalline upon cooling. The affect on nucleation of phthalocyanine pigments is disclosed in Gilbert et al., U.S. Pat. No. 3,558,551. While the effect of shrinkage can be compensated for by using a slightly larger mold or different machine set-up, the shrinkage attributable to the pigment may vary from one color of pigment to the next. Consequently, it may be very difficult for a parts manufacturer to provide an entire color line of opacified closures without having to change the molding equipment with each change in pigment.